The present invention is directed to a method for leak testing closed containers with at least one flexible wall area and to a leak testing apparatus for leak testing a closed container with such flexible wall area, irrespective whether such container is filled with a product or not.
When testing closed containers one known technique is to arrange a container to be tested into a test cavity which is then sealingly closed, then to evacuate the interior space of the test cavity around the container to be tested and to evaluate the time behaviour of the pressure in the surrounding of the container after evacuation has been stopped at a predetermined level. Although this technique is of very high accuracy it necessitates utmost care for reaching such high accuracy. The volume of the test cavity and its shape must snugly fit the outside shape of the container to be tested. On one hand minimising this volume leads to respectively short evacuation time, on the other hand the degree of this minimising largely governs the detection accuracy reached. As a change in pressure in the surrounding of the container is detected as leak indication entity, the smaller than the volume is in which, through a leak, pressure is affected, the higher will be the detection accuracy.
Further, accuracy is largely influenced by the degree of vacuum which is established in the surrounding of the container, which makes it necessary, for high accuracy, to provide relatively expensive vacuum pumps, possibly even multiple stage vacuum pumps, if vacuum is to be established down to the level as only reached with turbo vacuum pumps.
It is an object of the present invention to provide for a method and apparatus as mentioned above, which remedies for the drawbacks of state of the art leak testing technique using pressure monitoring. This object is resolved by the method of leak testing as mentioned above, comprising the steps of relatively moving a biasing member towards and onto the flexible wall area of the container, stopping such moving and monitoring a biasing force on said container. The biasing force monitored is sampled at a first point in time resulting in a first force measuring signal and is sampled at least one second subsequent point in time, resulting in a second force measuring signal. There is further generated a difference signal in dependency of said two measuring signals as a leak indicative signal.
Thereby, the present invention departs from the recognition that if a container to be tested is biased, leading to either compression or expansion of such container, biasing forces will apply to surfaces applied externally to the wall of the container as reaction forces of the expanded or the compressed container. Such reaction forces may easily be monitored. If such biasing is installed to a predetermined level and then stopped, a tight container will lead to monitoring a constant reaction force according to the biasing level reached. If the container is leaky, there will occur an exchange of medium between the surrounding of the container and its inside, leading to a decrease of the reaction force monitored over time.
Thereby, the accuracy of such a technique is largely independent from the volume surrounding the container under test, and is further primarily given by the degree of biasing and the force detecting surface towards which the biased container reacts.
In a preferred embodiment of the inventive method, biasing is installed up to a predetermined biasing force.
Having reached such predetermined biasing force, it is further proposed to wait for a time span before by sampling the respective first and second force measuring signals are generated, in dependency of which the difference signal is generated. Thereby, in this time span the biased container can stabilise its shape. In one operating embodiment biasing of the container under test is controlled as a function of the difference signal generated, so as to hold said difference signal on a predetermined value and exploiting the action of the biasing member as a leak indication. Thereby, a negative feedback loop is established, where the biasing member controllably counteracts a change of force monitored due to leakage, so that in extreme no change of force will occur due to the fact that the biasing member maintains by appropriate action a constant reaction force.
In a most preferred embodiment biasing the container is not established by relatively moving external surfaces onto the wall of the container, but in that a pressure difference is installed between the inside of the container and its surrounding. Thereby, the pressure difference is in a most preferred embodiment established by evacuating the surrounding of the container. The flexible wall area of the container has then the tendency of bowing outwards, and if this bowing outwards is barred by stationary surfaces outside the container, the container will act with a respective force on such surfaces. This force is monitored.
So as to avoid that due to the inventively exploited biasing, an existing leak in a container is clogged by the wall area with such leak being urged onto an external surface, it is proposed to provide surface areas contacted by the wall of the container, as it is biased, with a structure. Such a structure may be realised by interposing a mesh- or grid-like member between wall area of the container and such an external surface or, and preferably, by roughening such surface as by etching or machining.
In a further preferred embodiment the first force measuring signal is stored and the difference signal is generated in dependency of the stored first force measuring signal and the second measuring signal.
In a further preferred mode of operation, already in the first point in time there is generated the difference signal namely from the first force measuring signal stored, and the first force measuring signal unstored. The resulting difference signal, as a zero offset signal, is stored and zero offset of latter generated difference signal is compensated by the stored zero offset signal.
So as to early detect large leaks, then smaller leaks, it is further proposed to compare the biasing force monitored with at least one predetermined threshold value, at the latest when sampling at said first point in time, which leads to identifying very large leaks and further preferably to compare the difference signal with at least one predetermined threshold value.
The leak testing apparatus according to the present invention comprises a biasing arrangement for compressing or expanding a container under test, further a force detector applicable to the wall of the container under test and generating an electric output signal. The output of the force detector is operationally connected to a storing unit, the output of the storing unit operationally connected to a comparator unit. The second input of the comparator unit is operationally connected to the output of the force detector.
The invention is especially suited for leak testing so-called pouches, all around flexible wall containers, filled e.g. with pasty material.
Further preferred forms of realising the inventive method and the inventive apparatus will be become apparent to the skilled artisan reading the following detailed description as well as the claims.